1. Field of the Invention
The present invention relates to a pulse transmission time measuring apparatus that measures pulse transmission time, and a biological state estimating apparatus including the pulse transmission time measuring apparatus.
2. Description of the Related Art
Pulse wave velocity measuring apparatuses are known in the art to measure pulse wave velocity (or pulse transmission time) as information related to values such as the rate and time of propagation of a pulse wave in the arteries of a living body. The pulse wave velocity represents information related to the rate at which a pulse wave propagates between two areas of a living body. Thus, measurement of pulse wave velocity (or pulse transmission time) requires attaching two sensors to a living body to obtain measurements of a biological signal, such as an electrocardiographic waveform (electrocardiogram) and arterial pulse waves, at two different locations.
Japanese Unexamined Patent Application Publication No. 2004-321253 discloses a pulse wave velocity measuring apparatus that enables measurement of central pulse wave velocity and peripheral pulse wave velocity. The pulse wave velocity measuring apparatus includes the following components on its surface that is pressed toward the wrist: a semiconductor pressure-sensitive element that is pressed from above the epidermis toward the radial artery to detect radial artery pulse waves, and a piezoelectric sheet that is pressed toward the radial artery. The pulse wave velocity measuring apparatus also includes a probe attached to the fingertips to detect peripheral pulse waves. Further, the main body of a telephone includes a cardiac sound extracting unit that extracts cardiac sound from a signal output from the piezoelectric sheet, a central pulse wave velocity calculator that calculates the pulse wave velocity (central pulse wave velocity) from the heart to the wrist based on the extracted cardiac sound and the radial artery pulse waves, and a peripheral pulse wave velocity calculator that calculates the pulse wave velocity (peripheral pulse wave velocity) from the wrist to the fingertips based on the radial artery pulse waves and the peripheral pulse waves.
In the pulse wave velocity measuring apparatus, after the probe is attached to the epidermis of the fingertips, the pressing surface of a biological signal sensor unit is pressed against the wrist such that the pressure sensitive element disposed on the pressing surface presses against the radial artery, and the piezoelectric sheet disposed on the pressing surface presses against the radius. In this state, radial artery pulse waves are detected by the pressure sensitive element, vibrations including cardiac sound are detected by the piezoelectric sheet, and peripheral pulse waves are detected by the probe. The radial artery pulse waves, the cardiac sound, and the peripheral pulse waves are thus measured simultaneously. Then, the central pulse wave velocity is calculated from the cardiac sound and the radial artery pulse waves, and the peripheral pulse wave velocity is calculated from the radial artery pulse waves and the peripheral pulse waves.
Pulse wave velocity is measured as described above in the following manner by using the pulse wave velocity measuring apparatus described in Japanese Unexamined Patent Application Publication No. 2004-321253 First, the main body of the probe is attached to the fingertips, and the input terminal of the probe is inserted into the main body of the telephone. Then, with the cellular phone gripped in such a way as to touch a second electrode with one hand (for example, the right hand), the cellular phone needs to be pressed against the wrist of the other hand (for example, the left hand) such that a first pressing surface presses against the radial artery from above the epidermis and, at the same time, a second pressing surface presses against the radius from above the epidermis.
Thus, the pulse wave velocity measuring apparatus requires use of both hands to measure pulse wave velocity. This makes it difficult to obtain continuous measurement of pulse wave velocity during activities (for example, during daily life). Further, the attachment of the apparatus is cumbersome as described above, resulting in relatively poor ease of use.